In NSX version 4.2.1, when vIDM is configured for authentication, you observe the following login behavior:
This behavior differs from previous releases where you were automatically redirected to the vIDM login page upon accessing the NSX UI URL.
VMware NSX 4.2.1
This is an expected behavior starting with VMware NSX 4.2.1. You must now manually click the "Sign in with vIDM" link on the NSX login page to be redirected to the vIDM login page.